For example, in an image forming apparatus using an electrophotographic system, such as a facsimile, a printer or a copying machine, a toner is adhered to an electrostatic latent image formed on the surface of a photosensitive member to visualize it, the visualized image is transferred to a sheet or the like and thereafter it is fixed by being heated and pressurized. In terms of achieving high image quality and colorization, as a developer, a so-called two-component developer containing a carrier and a toner is widely used.
In a development system using a two-component developer, a carrier and a toner are agitated and mixed within a development device, and the toner is charged by friction so as to have a predetermined amount. Then, the developer is supplied to a rotating development roller, a magnetic brush is formed on the development roller and the toner is electrically moved to a photosensitive member through the magnetic brush to visualize an electrostatic latent image on the photosensitive member. The carrier after the movement of the toner is separated from the top of the development roller, and is mixed again with the toner within the development device. Hence, as the properties of the carrier, a magnetic property for forming the magnetic brush, a charging property for providing desired charge to the toner and durability in repeated use are required.
Hence, a carrier which is obtained by coating, with a resin, the surface of magnetic particles such as magnetite or various types of ferrites is generally used. In the magnetic particles serving as a carrier core material, not only a satisfactory magnetic property but also a satisfactory friction charging property for the toner is required. As the carrier core material which satisfies the properties described above, carrier core materials having various shapes are proposed.
The present applicant also focuses on a grain which forms a concave-convex shape of the surface of a carrier core material, that is, a difference between the peak part and the trough part of a so-called crystal grain and the shape of the corner of the grain, and proposes that the maximum height Rz which serves as an index for a difference between the peak part and the trough part of the grain and a skewness Rsk which serves as an index for the degree of deviation of a region of the peak part and a region of the trough part in the concave-convex portion of the particle surface be made to fall within specific ranges (patent document 1).